This paper reports on the combination of solid oxide fuel
cell generators fueled with biogas as renewable energy source, recoverable from
wastes. The solid oxide fuel cells have gained much importance in the recent
years for residential fuel systems. SOFCs could improve and promote the
exploitation of biogas on manifold generation sites as small combined heat and
power especially for farm and sewage installations raising the electrical
conversion efficiency on such reduced power level. The design of an independent
stationary residential fuel cell system with a generation capacity of 5 KW along
with the process of producing biogas from animal waste is studied. This document
compiles and estimates the biogas data that is required for the production of
hydrogen to supply the fuel cells and presents the thermodynamics and
electrochemical conversion processes. This paper also presents the power
conditioning system of the fuel cell system to provide the required voltage and
power to the application.

INTRODUCTION

The interest in the distributed generation has increased
significantly in the recent years. It is believed that the distributed
generation market will be between US $10 and $30 billion by 2010. Due to
environmental concerns, more effort is now being put into the clean distributed
power like geothermal, solar thermal, photovoltaic, and wind generation, as well
as fuel cells that use hydrogen, propane, natural gas or other fuels to generate
electricity without increasing pollution.

There are five major types of the fuel cells in current
technology. Among these five, Alkaline Fuel Cells (AFC) have been used in the
NASA space program since 1960s. Polymer Electrolyte Membrane (PEM) fuel cells
have very fast slew rates and low operating temperatures and are being used in
electric vehicles. Phosphoric Acid Fuel Cells (PAFC) are very tolerant to
impurities in the fuel steam and by far are the most mature in terms of system
development and commercialization. Over 200 stationary units with typical
capacity of 200 kW have been installed in the United States. Molten Carbonate
Fuel Cells (MCFC) and Solid Oxide Fuel Cells (SOFC) both operate at high
temperature 600-1,0000C, and are targeted at medium- and large-scale
stationary power generation. In SOFC, a solid ceramic material is used for the
electrolyte, and viable fuels can be used without a separate reformer. The
byproduct: hot water and heat can be used for heating. Much research has been
done towards the residential application of SOFC. One of the major obstacles of
its commercialization is the high cost of installation. In recent years, the
production costs of fuel cells keep decreasing.

Rising energy prices, broader regulatory requirements, and
increased competition in the marketplace are causing many in American
agriculture's livestock sector to consider anaerobic digestion of animal wastes.
They view the technology as a way to cut costs, address environmental concerns,
and sometimes generate new revenues. While hundreds of anaerobic-digestion
projects have been installed in Europe and the U.S. since the 1970s, it was not
until the 1990s that better designed, more successful projects started to come
on line in the U.S. Today, there are an estimated 40 farm-scale projects in
operation on swine, dairy, and poultry farms across the country. This paper
studies the process of production of biogas through anaerobic digestion and the
conversion of biogas to hydrogen and the production of electricity from hydrogen
by fuel cells. This paper calculates the amount of biogas needed and the amount
of manure required for producing 5 KW power from the fuel cells. The total study
can be briefly given as below.

Animal Waste (Biomass) ΰ
Methane (Biogas- along with other gases)

Methane ΰ Hydrogen (fuel for
fuel cells)

Hydrogen ΰ Electricity (through
fuel cells)

ANAEROBIC DIGESTION

Anaerobic digestion works in a two-stage process to decompose
organic material (i.e., volatile solids) in the absence of oxygen, producing
bio-gas as a waste product. In the first stage, the volatile solids in manure
are converted into fatty acids by anaerobic bacteria known as "acid formers." In
the second stage, these acids are further converted into bio-gas by more
specialized bacteria known as "methane formers." With proper planning and
design, this anaerobic-digestion process, which has been at work in nature for
millions of years, can be managed to convert a farmer's often problematic
waste-stream into an asset. The Figure 1 shows the basic components of the
anaerobic digestion process to produce biogas.

The key by-products of anaerobic digestion include digested
solids ( useful as a soil amendment) and methane, the primary component of
"bio-gas," which can be used to fuel a variety of cooking, heating, cooling, and
lighting applications, as well as to generate electricity. Capturing and using
the methane also precludes its release to the atmosphere, where it is 20 times
more damaging to the ozone layer than carbon dioxide.

Figure 1. Basic components of an anaerobic-digestion system

Source: ATTRA - National Sustainable Agriculture
Information Service,

http://attra.ncat.org/attra-pub/anaerobic.html

The plug flow type of anaerobic digester is the most commonly
used digester. This consists of a cylindrical tank in which the gas and other
by-products are pushed out one end by new manure being fed into the other end.
This design handles 11-13% solids and typically employs hot-water piping through
the tank to maintain the necessary temperature. Most appropriate for livestock
operations that remove manure mechanically rather than washing it out.

Procedure

The digestion process is not difficult but requires long
period of time. The digester tank is filled with water and then heated to the
desired temperature. "Seed" sludge from a municipal sewage treatment plant is
then added to about 15% of the tank's volume, followed by gradually increasing
amounts of fresh manure over a three-week period until the desired loading rate
is reached. Assuming that the temperature within the system remains relatively
constant, steady gas production should occur in the fourth week after start-up.
The bacteria may require two to three months to multiply to an efficient
population.

Temperature within the digester is critical, with maximum
conversion occurring at approximately 95°F in conventional mesophilic digesters.
For each 20°F decrease in temperature, gas production will fall by approximately
50 percent. Even more significant is the need to keep the temperature steady.
Optimal operation occurs when the methane formers use all the acids at
approximately the same rate that the acid formers produce them. Variations of as
little as 5°F can inhibit methane formers enough to tip the balance of the
process and possibly cause system failure.

Because of the extreme cost and difficulty of liquefying bio-gas, it
is not feasible for use as a tractor fuel. Bio-gas has many other on-farm
applications, like cooking, heating (space heating, water heating, and grain
drying), cooling, and lighting. Bio-gas can also be used to fuel generators for
producing steam and electricity.

While methane is a very promising energy resource, the
non-methane components of bio-gas tend to inhibit methane production and, with
the exception of the water vapor, are harmful to humans and the environment. For
these reasons, the bio-gas produced should be properly "cleaned" using
appropriate scrubbing and separation techniques.

Digester Design Factors

Digesters are installed primarily for economic and
environmental reasons. Digesters represent a way for the farmer to convert a
waste product into an economicasset, while simultaneously solving an
environmental problem. Under ideal conditions, an anaerobic-digestion system can
convert a livestock operation's steady accumulation of manure into a fuel for
heating or cooling a portion of the farm operation or for further conversion
into electricity. The solids remaining after the digestion process can be used
as a soil amendment, applicable on-farm or made available for sale to other
markets.

Anaerobic digestion requires careful consideration of many
factors. They can be quite costly to install. A straightforward batch-loading
design will involve an air-tight tank, means of mixing the contents of the tank
and maintaining a constant temperature, and a means of collecting the gas with
appropriate safety precautions. Additional hardware will include regulators,
flame traps, pressure gauges and relief valves, a hydrogen-sulfide scrubber, and
means of removing the carbon dioxide. The size of the system is determined
primarily by the number and type of animals served by the operation, the amount
of dilution water to be added, and the desired retention time. The most
manageable of these factors is retention time; longer retention times mean more
complete breakdown of the manure contents, but require a larger tank.

BIOGAS TO HYDROGEN CONVERSION

Methane steam reforming (MSR) is a major route for the
industrial production of H2. The three main reactions in a MSR
reactor are represented by following equations.

CH4 +H2O ΰ
CO + 3H2; ∆H298 = 206.2 Χ 103 kJ/kmol;
. (1)

CO+H2O ΰ CO2 +H2;
∆H298 = −41.1 Χ 103 kJ/kmol; (2)

CH4 + 2H2O ΰ
CO2 + 4H2; ∆H298 = 164.9 Χ 103
kJ/kmol; .. (3)

Fig 2: Steam Methane Reforming Process in a Fuel Processor

Source: [3]"Hydrogen from Methane in a single step
process", Chemical Engineering Science

Reforming reactions (1) and (3) are highly endothermic and
thermodynamically favored by high temperature and low pressure. On the other
hand, the watergas shift (WGS) reaction given by (2) is favored at low
temperature, but it has no pressure dependence. MSR is generally operated at a
temperature of 750900oC due to the overall endothermic nature of the
reactions. Although high-temperature operation is indispensable for a
substantial conversion of CH4, it facilitates the reverse WGS
reaction, giving the product gas containing 810% CO on a dry basis. For the
purpose of obtaining the product gas with less CO and more H2, it is
conventional that the MSR product gas is fed to another reactor where the
temperature is kept as low as 300400oC for the WGS reaction to take
place prior. To obtain the H2 product stream, the effluent is then
cooled and fed to a multicolumn pressure swing adsorption (PSA) process.

DESIGN OF COMPLETE FUEL CELL SYSTEM

Numerous calculations are involved in the design of a 5 KW
residential fuel cell system. It is desired to find the number of cows (cattle)
required to produce 5KW power, the size of the digester tank, the amount of
manure and biogas produced, the retention time of the manure in the digester
tank, the heat energy involved in the conversion of methane to hydrogen, the
volume of methane required to produce sufficient amount of hydrogen to produce 5
KW electricity.

The generating capacity of a typical hydrogen fuel cell is
167 watts at a voltage output of 1 volt. For higher output and terminal voltage,
cascades of series- parallel connections of fuel cells- will meet the required
electric load demands.

For a 5 KW residential cell system, the number of fuel cells
required in a stack is

The current passing through the fuel cell stack is I =
5000/30 ≈ 167 A .. [2]

It is known that current is the rate of charge. Hence

I = (n * e-)/t . [3]

where n: no. of electrons passing through the external load
connected to the fuel cell.

e-: charge of an electron = 1.602*10-19
coul

t: time of discharge of the electron

*Note: The time of discharge of the electron is the time
taken by the electron to travel from one electrode of the fuel cell to the
other. The time of discharge depends on the distance between the electrodes. The
time of discharge of a typical hydrogen fuel cell is

t = 1.0813 * 10-17 sec.

Hence the number of moles of electrons (from [3])

n = I *t / e- = (166.67 * 1.0813*10-17)/
1.602*10-19

n = 11249.70 moles of electrons

The electrochemical reactions taking place in the fuel cell
are

At anode A, 2H2 = 4H+ + 4e-
[4]

At cathode K, 4H+ + 4e- + O2ΰ 2H2O . [5]

And the overall reaction is

2H2 + O2ΰ
2H2O ... [6]

Hence from [4] it can be taken that the molar ratio of H2
to the electron is 1: 2.

Hence the number of moles of H2 ,

nH2 = 11249.70/2 = 5624.85 moles of H2

The reactions occurring in the conversion of methane to
hydrogen are

CH4 +H2O ΰ
CO + 3H2;

CO+H2O ΰ CO2 +H2;

The overall reaction is

CH4 + 2H2O ΰ
CO2 + 4H2;

From this it can be observed that the molar ratio of methane
to hydrogen is 1: 4.

*Note: It is taken that the complete chemical reactions
take place in the fuel reformation i.e., conversion of methane to hydrogen. The
rates of chemical reactions are considered when the dynamic model of the fuel
cell system is designed. The rates of reactions depend on the partial pressures
of the reactants and the products. The dynamic model of the fuel cell system is
required to analyze the transient changes in the electrical load. To study how
fast the fuel cell can respond to the sudden changes in the load can be
determined by the dynamic model. The time of response of the fuel cell system
depends on the rate of change in the input to the fuel cell (i.e., hydrogen).
This in turn depends on the rate of reactions of the fuel processor.

35 % of the biogas produced is used to maintain the
temperature in the tank.

POWER CONDITIONING SYSTEM

This paper uses an inverter system that supports the
commercialization of a 5 kW solid-oxide fuel cell (SOFC) power generation system
to provide non-utility and ultra-clean residential electricity. The aimed fuel
cell outputs 22 V to 41 V dc. For residential applications, the needed output is
two split-phase 60 Hz, 120 V ac, the 5 kW SOFC is supplemented with a 5 kW
battery pack to meet peak power-demand of 10 kW. The general inverter system
configuration is shown in Fig. 3. The dc voltage from the fuel cell is first
boost up to 350-450 V by a dc-dc converter then a dc-ac inverter with output
filter is cascade-connected to produce ac voltage. The battery can be added to
either the low voltage side or the high voltage side of the converter.

Fig 3: Components of Power Conditioning System of a Fuel Cell system

Source: [5] A New Low Cost Inverter System for a 5 KW fuel
cell

The dc-dc converter uses phase shifting to control power flow
through a transformer with a full bridge on the low voltage side and a
controlled voltage doubler on the high voltage side. The transformer provides
voltage isolation between the fuel cell and the ac output voltage improving
overall safety of the system. A voltage doubler on the high voltage side
decreases the turns ratio of the transformer, which reduces leakage inductance
and makes the system more efficient and easier to control. And at the same time,
the voltage and current stresses on the low voltage side are also minimized. A
high voltage battery pack is added after the voltage doubler as transient power
for load dynamics. Thus the capacitance of the high voltage side capacitors are
minimized, which will significantly reduce the total cost of the system.

The overall residential fuel cell system is shown in the
figure 4.

CONCLUSION

This paper mainly concentrated on the design issues of an
independent residential fuel cell system. The power generation system is not
connected to the grid but it supplies power to the farm house. A typical 5 KW
fuel cell system is taken into consideration. It is calculated that 96 cows are
required to yield 1200 gallons of manure per day. This manure when subjected to
anaerobic digestion for 15 days in a cylindrical 18 feet diameter , 18 feet tall
tank, it produces biogas which consists of 60 -70 % of methane, the rest
consisting of carbon dioxide, oxides of nitrogen and other gases. This biogas is
sent to the fuel processing system which converts methane into hydrogen. It is
noted that 99% of methane is converted into hydrogen. This is a highly
endothermic reaction which requires a temperature of 800o C. The
hydrogen obtained is passed into a fuel cell stack consisting of 30 fuel cells
in series. The output voltage of the fuel cell stack is 30 V with a power of 5
KW. The obtained power is in DC form. This is converted into 120V ac split phase
power by a power conditioning system consisting of a dc-dc boost converter
followed by an inverter. An auxiliary power supply of 5KW is taken by a battery
connected to the low voltage side of the dc-dc converter.